1. Field of the Invention
This invention relates to an image pickup device having an index signal generator.
2. Description of the Prior Art
Various types of image pickup devices with an index signal generator are known which produce image signal including index signals for deriving accurate synchronous signals for controlling electron beam scanning speed or spot position or for providing timing for color-multiplexed video signal demodulation. The index signal generator which is provided to an image pickup divice produces index signals. The generated index signals correspond to timing or an instantaneous position of scanning electron beam which is provided for detecting photoelectrostatic image produced from a projected optical image.
Such index generator is normally provided behind of a faceplate of an image pickup device sharing with a color stripe filter on the surface of the faceplate. The index signal generator has a stripe pattern screen, illuminating means, and light shield means. The stripe pattern screen is placed behind the faceplate. The stripe pattern covers two peripheral portions of a photosensitive layer. One peripheral portion is at the beginning of each horizontal electron beam scanning, i.e., a left end of a raster when viewed from the front thereof; the second peripheral portion, at the beginning of vertical electron beam scanning, i.e., a top end of the same raster. Additional stripe patterns may be provided as options on the other peripheral portions. The stripe pattern is arranged in such a way that electron beam scanning line perpendicularly intersects opaque bars of the stripe pattern. The stripe pattern screen is provided in the path of incident light to a photosensitive layer of the image pickup tube. A light source which illuminates the stripe pattern screen is provided so that the stripe pattern is projected to the photosensitive layer. On the other hand, an optical image is focused onto the photosensitive layer. The resulting image produces a photoelectrostatic image on the photosensitive layer. The photoelectrostatic image is detected by electron beam scanning to generate a signal of the optical image togeter with a signal of the stripe patterns which represents timing of the electron beam scanning. Thus, the resultant signal has optical image information and stripe pattern information. The stripe pattern signals are used for synchronizing the optical information signal with other signal or for scanning calibration purposes. For example, U.S. Pat. No. 4,736,243 discloses such a technique for single-tube color image pickup devices. In this color image pickup device, an optical image is focused on a photosensitive layer of a color image pickup tube through a color stripe filter having a plurality of successively arranged recurrent groups of different color stripes and converted into an electrostatic image which is scanned in rectangular raster form by an electron beam to generate a color-multiplexed video signal. First and second index stripe bars are located adjacent to edges of the rectangular raster scan area to generate first and second index signals. Prior to normal imaging, the photosensitive layer is illuminated uniformly with light of a predetermined color to generate a reference "carrier" which is stored in a field memory together with the index signals. During read mode, a carrier is generated having a component modulated in phase with the stripes of each recurrent group, and a component modulated in amplitude with the intensity of the picture elements of the image. The memory is read to generate a reference carrier as well as first and second reference index signals. These index signals are compared by comparators in frequency and phase with the corresponding index signals from the photosensitive layer and stored in sample-and-hold circuits to control the frequency and phase relationships between the modulated and reference carriers.
However, in such imaging devices with an index signal generator, there is a shading problem that a part of the projected stripe pattern onto the photosensitive layer, which part is nearest to the light source of the illuminating means is brightest and any other part becomes darker progressively as the distance from the brightest point increases. One reason is that the distance between the stripe pattern screen and the light source is relatively small so that variation of the distance is great. Another reason is as follows:
A single light source having a small light emitting spot generates sharp edge image of the stripe pattern on the photosensitive layer which generates accurate timing of the index signal. However, such a small light emitting spot naturally makes shading.
Accordingly, the level of index signal output varies in accordance with variation of brightness over a stretch of the stripe patterns. An index signal with low output level causes a low S/N ratio because an index signal processing circuit is designed to process the index signal which is expected to be its maximum level. In the above-mentioned color image pickup device, the index signal with level variation causes phase shift in color demodulation, i.e., color-demodulation error. Therefore, such index signal level variation causes unevenness of color to occur in vertical scanning direction on the resultant repoduced image.
There is also a technique where the index signal is used for controlling raster position, such as centering, in a monochrome or color image pickup device. In this device, the index signal is used for controlling start timing and speed of electron beam scanning. In such a device, the level variation of the index signal causes underscanning or oversccaning of the resultant raster.
Therefore, in the prior art image pickup device with an index signal generator, there is a drawback that the undesirably level varying index signal deteriorates the quality of a reproduced picture.